As used herein GTL-process means a process for production of hydrocarbons requiring a H2/CO molar ratio of 2.3 or below, such as a process for production of diesel via Fischer-Tropsch synthesis in which synthesis gas is converted into liquid hydrocarbons via Fischer-Tropsch reactions, or equivalent process such as a process for production of gasoline in which synthesis gas is first converted to oxygenated compounds in the form of methanol and dimethyl ether (DME) and subsequently to gasoline as disclosed in U.S. Pat. No. 4,520,216 and U.S. Pat. No. 4,481,305.
Particularly for Fischer-Tropsch operation the synthesis gas is required to have a H2/CO molar ratio of 1.7 to 2.3. It is known that these values are compatible with those obtainable when utilizing ATR or CPO in the synthesis gas section while running in continuous operation using recycled off-gas from downstream unit, such as a Fischer-Tropsch synthesis unit, as part of the feed. However, we have found that upon the starting up of such processes where off-gas is not available, the H2/CO molar ratio is above 2.3 for typical natural gas.
EP-A-0287238 discloses a process for the production of hot pressurised hydrogen-containing stream in which a recycle portion of the hot product gas stream from a catalytic partial oxidation reactor is combined with the gas feed in order to preheat the gas fed to the combustion catalyst, thereby enabling easier catalytic combustion. The recycle gas contains not only hydrogen, but also steam.
EP-A-1400489 discloses also a process for the production of synthesis gas in which a portion of the effluent from an autothermal reformer is recycled into the hydrocarbon-steam mixture entering the autothermal reformer by means of a thermo-compressor ejector which uses said mixture as motive fluid. The recycle contains not only hydrogen, but also steam to enable soot-free operation of the autothermal reformer and thereby avoid plugging of the catalyst bed. WO-A-2005/000736 discloses also a reforming process in which a pre-reformed hydrocarbon feed is added to an autothermal reformer and part of the effluent gas is recycled to the autothermal reformer.
It is also known to recycle a portion of the effluent stream in the form of CO2 from an autothermal reformer into the natural gas fed to the autothermal reformer. The effluent stream from the autothermal reformer is dehydrated first and then passed through a CO2-removal unit in order to form a synthesis gas stream and a CO2-stream. The CO2-stream is recycled and mixed with the natural gas upstream the autothermal reformer.